The present art to optical switches extract part of signal light for output and optical waveform monitoring apparatuses monitor waveforms of light to be measured using the optical switches as optical sampling gates. In particular, the present art relates to an optical switch for extracting signal light by utilizing nonlinear effects caused by control light and an optical waveform monitoring apparatus.
There is a technique, as one of basic techniques for observing optical signals, in which optical signals are converted into electric signals using optical receivers and then the electric signals are observed on electrical oscilloscopes. FIG. 6 illustrates an example of an optical waveform monitoring apparatus which has been widely used. In the optical waveform monitoring apparatus illustrated in FIG. 6, light to be measured (optical pulse) is converted into an electric signal through a photoelectric converter 101. Then, an electrical sampling pulse generating circuit 103 generates an electrical sampling pulse in accordance with an electrical trigger generated by a trigger circuit 102. In accordance with the electrical sampling pulse, the electric signal from the photoelectric converter 101 is sampled in a sampling circuit 104, and a resultant sampling waveform is displayed on a waveform display 105. In such an optical waveform monitoring apparatus, the actual waveform of an optical signal is sampled using an electric signal for waveform monitoring. This configuration provides the optical waveform monitoring apparatus with high stability.
However, such a common optical waveform monitoring apparatus described above has a problem that the time resolution is limited by the bandwidths of the photoelectric converter 101, the trigger circuit 102, the sampling circuit 104, the waveform monitor 105, and so forth. In general, the operation bandwidth of an electronic circuit is approximately 40 gigahertz (GHz), and thus it is difficult to achieve a time resolution higher than 10 pico-seconds (ps).
To overcome the above problem, an optical sampling technique samples signal light without photoelectric conversion. In this optical sampling technique, as illustrated in FIG. 7 for example, an optical sampling gate 111 composed of a nonlinear medium generates intensity correlation signal light corresponding to light to be measured having a repetition rate of f0 (wavelength: λs) and an optical sampling pulse of which the pulse width is narrower than that of the light to be measured (wavelength: λc), such that a sampling result is output. The intensity correlation signal light is herein referred to as light generated due to overlap of the light to be measured and the optical sampling pulse in the time domain.
Specifically, in the above configuration example illustrated in FIG. 7, a short pulse light source 112 is driven by a sampling frequency signal supplied from a sampling frequency generator 113, so that an optical sampling pulse can be obtained as output of the short pulse light source 112. In the sampling frequency generator 113, a clock signal at a frequency of f0 synchronized with the light to be measured is frequency-divided by N and is frequency-shifted by a frequency Δf for sweep, and a signal at the resulting frequency (f0/N+Δf) is output as a sampling frequency signal.
Then, the optical sampling pulse and the light to be measured are multiplexed in the optical sampling gate 111 and input to the nonlinear medium of the optical sampling gate 111, and as a result a nonlinear effect is produced. Through this nonlinear effect, an optical signal having signal having an intensity correlation between the light to be measured and the optical sampling pulse, i.e., intensity correlation signal light (repetition rate: f0/N+Δf) can be obtained. This intensity correlation signal light is extracted through an optical filter and output from the optical sampling gate 111.
The intensity correlation signal light output from the optical sampling gate 111 is converted into an electric correlation signal in the photoelectric converter 114. The converted signal is usually analog-digital converted using a sampling frequency signal and then input to a vertical axis signal port of the waveform display 115 as an intensity correlation signal converted into an electric signal. In the waveform display 115, a sweep signal at a frequency of Δf used in the sampling frequency generator 113 is input to a horizontal axis signal port and triggered, such that the waveform of the intensity correlation signal input to the vertical axis signal port is displayed.
Since the time resolution of the optical sampling described above depends principally on the pulse width of an optical sampling pulse, optical sampling with a high time resolution can be realized by preparing an optical sampling pulse having a narrow pulse width. With this arrangement, a time resolution sufficient to monitor a signal light waveform having a bit late of 40 gigabit/second (Gb/s) or higher can be achieved.
Optical sampling gates used in the above optical sampling technique have been developed. For example, an optical sampling gate configured to have an optical switch is proposed, as illustrated in FIG. 8 (see, for example, Japanese laid-open Patent No. 2006-184851, Japanese laid-open Patent No. 2006-194842 and S. Watanabe, et al., “Novel Fiber Kerr-Switch with Parametric Gain: Demonstration of Optical Demultiplexing and Sampling up to 640 Gb/s,” ECOC. 2004, Th4. 1.6).
In the optical switch in FIG. 8, signal light (light to be measured) having a wavelength of λs and control light (optical sampling pulse) having a wavelength of λc which is different from the signal light, are input to a directional coupler through respectively corresponding polarization controllers 121 and 122, respectively. Then, light multiplexed by the directional coupler 123 is supplied to a nonlinear medium 124. At this time, the polarization direction of the signal light to be input to the nonlinear medium 124 is controlled by the polarization controller 121 to be orthogonal to a transmission axis of a polarizer 125 arranged downstream of the nonlinear medium 124, as illustrated in the left side of FIG. 9. In addition, the polarization direction of the control light to be input to the nonlinear medium 124 is controlled by the polarization controller 122 to be angled at 40 to 50 degrees with respect to the polarization direction of the signal light.
When no control light pulse is present, as illustrated in the lower left portion of FIG. 9, the signal light input to the nonlinear medium 124 passes through the nonlinear medium 124 while maintaining the polarization state of incident light. Thus the input signal light is intercepted by the polarizer 124 having a transmission axis perpendicular to the polarization direction. On the other hand, when a control light pulse is present, as illustrated in the upper right portion of FIG. 9, the signal light is optical-parametric amplified in the polarization direction of the control light by the effect of four-wave mixing. Four-wave mixing is generated selectively with respect to signal light having the same polarization component as control light. Thus, when the peak power of a control light pulse is sufficiently large, the polarization direction of the signal light which has been optical-parametric amplified in the nonlinear medium 124 is approximately the same as the polarization of exciting light. Therefore, an optical signal component, among components of the signal light emitted from the nonlinear medium 124, which is parallel to the transmission axis of the polarizer 125 is transmitted through the polarizer 125. Then, the transmitted light from the polarizer 125 is supplied to a wavelength filter 126 to extract light having a wavelength of λs, such that signal light switched in accordance with the control light is output from the wavelength filter 126.
According to the optical switch described above, increased switching efficiency can be realized as compared with a known optical Kerr switch (for example, see K. Kitayama et al., “Optical sampling using an all-fiber optical Kerr shutter,” Appl. Phys. Lett., vol. 46, pp. 623-625, 1985.) by exploiting the optical parametric amplification effect, which is produced due to four-wave mixing in the nonlinear medium 124. It is also advantageous that it is not necessary to highly precisely control the power of control light as in the case of the optical Kerr switch.